The necessity of treating aggregates in fluid media is a problem common to a wide variety of industries. Various forms of aggregates must be dealt with and various techniques have been developed. Before discussing areas in which problems with aggregates in fluid media are encountered, certain terms used herein need to be defined.
The term "aggregate" as used herein means a mass or a body of units or parts associated--generally somewhat loosely--with one another. It includes such things as (1) gels, i.e., colloids in which the dispersed phase has combined with the continuous phase to produce a semi-solid material, (2) masses of solid particulates such as carbon black, pigments and the like in which individual particles are associated with one another to form a clump or clustered mass, and (3) masses of needle-like or elongated particles having relatively high aspect ratios which are associated with one another to form a clump or clustered mass. The latter category (3) includes needle-like materials such as metallic oxides used in the manufacture of magnetic tape.
The terms "disperse" and "dispersing" as used here with regard to the treatment of aggregates refer to the breaking up of aggregates to form smaller aggregates and in some applications the partial or substantially complete breakup of aggregates into their individual components, i.e., into the individual particles which collectively formed the aggregates.
A previously noted, the need to disperse aggregates gates in fluid media is a problem common to many industries. For example, hydroxyethylcellulose ("HEC") is widely used in oil well completion work for the economic preparation of viscosified brines, primarily to obtain plug flow during the last stages of cleaning of undesirable solid particles and gravel from the otherwise completed well. Viscosified brines are prepared by combining salts, such as alkaline and alkaline earth halides, e.g., sodium chloride and calcium bromide, with water to increase the density.
The compositions are made viscous by including a water soluble polymer, e.g., HEC. The quantity of polymer required to achieve the desired viscosity generally contains an undesirable level of gel aggregates. Gel aggregates in well completion fluids are undesirable for two principal reasons: (1) they tend to plug the filters used to clean up a completion fluid prior to its injection into a well, and (2) the gel aggregates are themselves highly deleterious to oil production if they are included in a fluid injected into the well since they tend to plug the formation. In order to obtain a useful brine or well completion fluid, then, the gel aggregates in the viscosified brine fluid must be removed and/or reduced to a fine state. This can be accomplished by filtration but the cost and time required are excessive due to rapid filter plugging Attempts have been made to reduce the gel aggregate content by other means but these have generally been accompanied by a quite large reduction in viscosity of the fluids, an undesirable side effect since the primary reason for adding the polymer is to increase the viscosity.
A system, then, capable of removing gel aggregates from such systems and/or reducing the size of gel aggregates to a fine state to alleviate filter plugging and reduce damage to oil bearing formations, particularly if that system were self-cleaning and did not substantially effect the bulk viscosity of the brine, would be highly desirable.
A second area in which aggregate formation and subsequent filter plugging causes problems is in the manufacture of high fidelity magnetic tapes and the like. Compositions used in the manufacture of such tapes generally comprise a mixture of (1) one or more metal oxides, such as oxides of chromium and iron, which typically are in the form of needle-like particles, and (2) a resin system, with this mixture dispersed in an organic liquid such as methyl ethyl ketone, toluene or the like.
Compositions of this type are prone to aggregate formation and subsequent filter plugging since the filters used are relatively fine to insure a uniform and fine level of dispersion of the metal oxide particles necessary for the manufacture of high quality, high fidelity tapes. Concomitantly, they are more susceptible to plugging. Typically, relatively expensive, porous stainless steel filters are used. The replacement cost when rapid plugging occurs, necessitating quick change-out, is quite high. The difficulties in filtering these types of systems are generally known. A filter with fine pores plugs rapidly although the product (effluent) is satisfactory. Alternatively, a more coarse filter has a longer onstream life but the resulting product is of lesser quality. To achieve both the desired economic life and an acceptable effluent is difficult. Compounding the problem, the resin system itself can contribute to the manufacture of an inferior product due to insoluble crosslinked polymeric gel aggregates formed during the normal manufacturing process for resins. If not removed, these gel-based aggregates, as well as oxide-based aggregates, interfere with the reproductive fidelity of magnetic tapes by creating background noise due to the resulting rough surface of the tape. A dispersion system then operating ahead of these filters to remove such aggregates and/or reduce their size would extend the life of the fine filters required and enhance the economics of the process.
In addition to the need for a high and uniform level of dispersion in such compositions, it is also required that destruction or breakdown of the individual needle-like particles, typically having relatively high aspect ratios, e.g., 10-15 to 1, be avoided. Accordingly, in both the initial formation of the suspension or dispersion used in magnetic tape manufacture and in the subsequent treatment of such dispersions, a self-cleaning system having the capability of both initially forming a uniform dispersion and subsequently insuring that it remain substantially free of aggregates would be highly desirable.
Another application in which the uniform dispersion of solid particulate matter in a fluid medium is desirable is in the dispersion of pigments such as carbon black and the like where the fine particles tend to agglomerate. Many compositions where solid particulates, such as carbon black and other pigments, are used also contain high molecular weight binders or thickeners which commonly contain undesirable gel-like aggregates. The system and method of this invention serve to disperse the solid particulate aggregates without substantial adverse effect on the properties of the binder or thickener. Indeed, the system and method of this invention also serve to reduce the undesirable gel-like aggregates commonly present in such systems. Typically, these compositions are used as paint bases and, in general, the higher the level of dispersion, the more effective a given weight of pigment, i.e., the more finely dispersed the pigment, the less that is required.
There are many other industries and applications where there is a need for an ability to provide a high and uniform level of dispersion of aggregates in a fluid, for example, in the spinning of fibers from polymers where gels can cause fiber breakage during drawing of the fibers and, similarly, in film casting and extrusion where gels can cause "fisheyes" due to local thickening of the film or, conversely, may cause holes in the film.
As described hereinafter, the system and method of this invention provide a straightforward, efficient and clean technique for dispersing aggregates in fluid media and, in large measure, overcome the problems heretofore only partially solved by prior art techniques.